The present invention relates generally to the field of non-impact printing and, in particular, the present invention reveals a method and apparatus for improving both color fidelity and registration among several non-impact print heads operating in a digital color print engine.
In the prior art related to ink jet printing a print head operated under precise electronic control typically opposes a portion of a printing media so that an image may be printed thereon. Typically, to achieve printed images of the highest quality each of a plurality of ink emitting elements that emit droplets of colorant onto the printing media need synchronization in respect of their position and orientation with respect to each other such element (i.e., exact xe2x80x9cregistrationxe2x80x9d). In prior art multihead digital print engines including drum-based, swath (or carriage-based), and flat-bed digital print engines, it is known that consistency of mounting and operation of such elements increases the level of registration among said elements and thus decreases the likelihood of printing errors and image artifacts. In a traditional drum-based print engine a print media attaches to a rotating drum which then passes under one or more discrete ink emitting print elements (xe2x80x9cnozzlesxe2x80x9d) mounted on a carriage articulated in the axial direction. In a flat bed print engine, the printing media is rigidly coupled to a substantially planar surface and the nozzles are articulated in two dimensions to cover the media. In a reciprocating swath, or carriage-based, print engine the media is incrementally stepped over a platen surface in one direction while the nozzles reciprocate across the media in a direction orthogonal to direction the media advances. In many of these traditional print engines perfect registration has become even more difficult to efficiently achieve as the number of print heads and the number of ink emitting elements increase and service and replacement procedures become more frequent. In each of these types of prior art print engine mechanisms, registration among and between nozzles of print cartridges may always be improved since no known means yet exists to rapidly, and perfectly, register each element to every other ink emitting element. Accordingly, in practical terms it is known that in some businesses specializing in producing full color digitally printed output, time constraints to complete printing jobs will conflict and oftentimes prevail with time required to complete full calibration and registration routines.
Furthermore, due to imperfection and general variation introduced during manufacture of print head elements, and their associated mounting elements, a number of electrical and mechanical variables that impede extremely accurate dot placement in an ink jet print engine thus compounds the difficulty in achieving perfect registration among all print heads at all times during printing operations. Particularly with reference to disposable ink jet print cartridges, xe2x80x9ccartridgesxe2x80x9d or xe2x80x9cprint headsxe2x80x9d herein, variations among cartridges are even further compounded as a result of periodic removal, substitution, cleaning, and/or replacement of a given one or more of several cartridges where misalignment error(s) regularly occur from inexact replacement following removal.
In these and other printing processes output is created by a plurality of multi-hued ink droplets emitted under precise electronic control in sequence from ink emitting nozzles of cartridges. Such ink droplets must record (a xe2x80x9cdotxe2x80x9d) as close as possible to exact pre-selected locations on the printing media to accurately reproduce printed output of an original source image with color fidelity and graphic quality corresponding to the original image. Unfortunately, due to a number of underlying causes, including compromises between time and quality in volume image production environments, said droplets often record dots upon the printing media at imprecise locations and thus generally degrade image quality and color fidelity of the printed image. As noted above, a primary cause involves a simple and oftentimes misalignment of one or more of the print heads (and thus the ink emitting nozzles associated with said print heads). In print engines that utilize disposable or removable print heads such slight misalignment potentially occurs every time one or more print heads is replaced or removed during periodic manual cleaning and other service of said print heads. Other causes of misregistration include differing ink droplet volume, varying ink droplet velocities of droplets emitted from different nozzles of a print head, bi-directional printing, slight non-alignment of the print heads, differing thickness of the printing media, and differing electrical characteristics of individual ink emitting nozzles and/or cartridges, among others.
Thus, it is known and can be appreciated that electrical and mechanical tolerance variations introduced during manufacture (and human error in mounting) of said cartridges has long presented, and continues to present, obstacles to extreme visual clarity in high speed digital color drop-on-demand and continuous-type printing. A clear implication of the level of compensation desired in the prior art is to allow for manufacturing tolerances to be relaxed somewhat without degradation in image quality, and thus manufacturing costs can decrease to the degree such tolerances can be relaxed.
Many prior art approaches to improving registration of a plurality of print heads, or compensating for image quality defects involve manual inspection, manual entry of perceived data values into an electronic print engine controller, and manual cleaning operations of each print head, although other varied approaches have been disclosed in the prior art. For example, in U.S. Pat. No. 5,644,344 issued to Haselby Jul. 1, 1997 depicts methods of calibrating and aligning an operation of print head cartridges in a swath printer using a carriage-mounted analog sensor oriented to sense edges of line segments printed by print cartridge print elements and then calculating a linear equation that transforms optical sensor values to adjust swath data shifts and timing delays. This representative prior art approach fails to account for a number of variables in printing that are addressed in the present invention, but otherwise adequately describes the state of the prior art fairly well.
Thus, a need exists in the prior art to solve issues related to the performance limitations of known print engines which emit ink from nozzles onto a print media. Further, a need exists in the art of digital ink jet printing to compensate for minute registration, or dot placement errors, and faulty performance of and among nozzles of print cartridges and to accurately sense and control registration and color image fidelity by sensing individual dots created by colored ink droplets in order to improve the quality and the visual clarity of text, graphics, and color appearing on the print media. Finally, a need exists in the art to improve the yield of quality digital output given practical and mechanical constraints imposed by use of ink emitting print heads mounted at some distance above a printing media as to synchronize and perfect registration among each of a plurality of colored ink droplets so they accurately record dots upon desired locations on the printing media to thus rapidly form high quality printed output closely resembling original source images.
The method and apparatus of the present invention increases the precision for controlling a plurality of cartridges that emit colored ink droplets from at least two ink jet print cartridges in a digital print engine. The present invention addresses both registration and color fidelity aspects of digital color ink jet print engines by utilizing an optical sensor to sense and accurately locate patterns of individual dots created by droplets emitted from said print cartridges. A focused source of illumination preferably periodically illuminates individual dots on the printing media so that such individual dots can be sensed by an array of optical sensor elements. A series of electronic images are recorded during said periodic illumination and each may be stored, compared to a corresponding series of reference dot patterns, then used for updating an electronic printing sequence, and/or viewed on a monitor to confirm orientation and location of the optical sensor with respect to individual dots. The electronic image is typically temporarily stored as a two dimensional bit map in a portion of a memory storage device that may include location, size, and color information of each individual dot interrogated and successfully detected by the optical sensor. The source of illumination may comprise many different colored source elements, such as red, green, blue (RGB); or cyan, yellow, magenta, black (CMYK); or other; depending on which color space is desired for color correction procedures as is known and used in the art.
The present invention thus finds increased utility over a variety of prior art printing methods and platforms to achieve both accurate placement and registration among a plurality of ink droplets recorded on a variety of desired pre-selected locations of a printing media and to confirm or correct color fidelity of an image. By sensing dot patterns produced by one or more print head cartridges with a first print nozzle control sequence and then determining which of a variety of controlled parameters to adjust to improve registration and color fidelity first among nozzles of each cartridge with respect to each other and thereafter among nozzles of different cartridges. In a preferred embodiment of the present invention, a print engine employs several print heads that can readily provide nozzle redundancy so that mis-firing and non-firing nozzles may be compensated and replaced by fully operational nozzles without degradation of image or needless loss of available printing time. The initial steps of the inventive method herein preferably include conducting compensation calculations based upon the location of discrete dots recorded on the print media, which calculations are promptly implemented in an amended excitation control sequence prior to initiating later calibration steps so that successively finer tuning for dot placement accuracy results.
The present apparatus includes an optical sensor for sensing and storing information about dots recorded on a print media by said print heads wherein the optical sensor is preferably coupled to the carriage assembly, and based upon each of several iterative steps where differing calibration patterns are optically sensed, achieves highly accurate registration among the print heads.
The following figures are not drawn to scale and only detail a few representative embodiments of the present invention, more embodiments and equivalents of the representative embodiments depicted herein are easily ascertainable by persons of skill in the digital imaging arts, and are expressly covered hereby. The inventors reserve the right to augment or otherwise render any portion of the written description, and those aspects inherent therein and known to those of skill in the art, as illustration(s) hereof.